New York landscape architect Thomas Balsley delights in promising to show friends what he calls “the greatest untapped open space opportunity in America.” He then takes them to the top of the Empire State Building and points to the ocean of rooftops visible to the horizon in every direction.

A city receives exactly as much sunlight and rainfall as it did before development when the area was a virgin forest or grassland, but now much of the meteorological action is off the ground on top of structures. Although most individual houses have sloping roofs that are perhaps better suited for solar collectors or conduits for rain barrels, most large institutional or residential buildings have flat ones that could potentially be used for parks. Rooftops represent the rare resource that is increasing. Most are private, but a significant number are publicly owned. And some of those are large–the tops of schools, libraries, government office buildings, post offices, concert halls, convention centers, parking ramps, and bus stations can all extend to well over an acre. Moreover, large private rooftops, such as those on shopping centers, big box stores, and warehouses, are purchasable or leasable, just like any other private property.

The green roof movement, still in its infancy, is mushrooming in popularity along with the green building movement in general. But merely being green does not make a roof a park. That oft-cited Chicago City Hall green roof is a marvelous creation but it isn’t a park, just as the much older green roof on the Time-Life Building in New York City isn’t a park–neither is freely open to the public. There are already thousands of rooftop gardens, pools, and recreational facilities on top of luxury condominiums, apartment buildings, office buildings, and hotels across the country, but they are private facilities for residents, workers, guests, and members only. They are the vertical equivalent of parks inside gated communities.

At present the majority of rooftop parks are ones at ground level built over subsurface parking garages–places like Millennium Park in Chicago, Hudlin Park in St. Louis, and Yerba Buena Garden in San Francisco. This wonderful urban solution is referred to, at Boston’s Post Office Square, as “Park Above, Park Below.” Older facilities are of relatively conventional ornamental design; new ones increasingly incorporate more ecological features. Both Nashville’s new Public Square and Austin’s new City Hall collect all the gathered rain for later use as pumped irrigation water.

Putting parks on rooftops higher than street level is, thus far, much rarer. For one thing, keeping the plant material alive is a challenge because of more extreme conditions of wind, sunlight, thin soil, and lack of trees. For another, there are concerns about the structural strength of buildings and potential water leakage as well as issues of human access and security. Also, what park uses are appropriate on rooftops? Flower gardens, lawns, benches, and pathways? Courts (surrounded by cages) for basketball, tennis, and volleyball? Community gardens? Playgrounds? Dog parks? Miniature golf?

These are complex questions that require a good deal of research–both into the issue of “rooftops” and of “parks.” Some of the investigation is generic, some of it needs to be highly specific, on a city-by-city basis. How many flat rooftops does, say, Miami have? What is the total combined acreage? How many are on public buildings and what is that combined acreage? How many of them are large, say, an acre or more? How many of those large ones are relatively uncluttered with air conditioning units and other paraphernalia? How many are accessible by the public? How many happen to be in areas underserved by parks? This entire field of inquiry is so new that there are almost no data available, although there will be breathtakingly quick progress now that aerial photography is so widely available through Internet programs.

One of the most famous, and one of the oldest, rooftop parks is NCNB Plaza (also called Kiley Garden) in Tampa, Florida. Rarely known for urban innovation, Tampa backed into its moment of leadership as a result of authentic civic outrage over the sale and destruction of its historic rose garden for the erection of a 33-story bank tower. In a creative (and expensive) gesture of repair, North Carolina National Bank agreed to restore the lost green space by putting a park on top of the new tower’s parking deck. Designed by (and named after) prominent landscape architect Dan Kiley and opened in 1988, the plaza is 8 feet above ground level.

Kiley Garden represents all that is great and all that is problematical about rooftop parks. On the positive side, it provides outstanding views of Tampa’s downtown and of the Hillsborough River and its riverwalk. It came into being in a high-land-value location which, under other circumstances, would never have yielded a public park. On the other hand, there were design and construction shortcomings that ultimately–nineteen years later–forced a total and complete renovation, including the removal of every tree, shrub, and blade of grass, as well as the many architectural elements. Of course, almost every aspect of technology has evolved in the past two decades, and the lessons of Kiley Garden can be used to help make rooftop parks more successful in the future.

The two places that have taken rooftop parks the furthest are New York and San Francisco–not surprisingly, since they are the two most crowded big cities in the country.

At 28 acres, New York’s Riverbank State Park is so large that it contains a pool, a skating rink, a theater, four tennis courts, four basketball courts, a wading pool, a softball field, a football field, four handball courts, a running track, two playgrounds, a weight room, a boat dock, and a restaurant. It was built on the roof of a new sewage treatment plant on the Hudson River and provides an exciting template of how large public buildings can be constructed to do double duty.

San Francisco has St. Mary’s Square, a full-fledged, off-the-ground, up-in-the-air park amid the high rises in the Financial District. Moreover, St. Mary’s Square will soon be getting an addition (on another rooftop) thanks to a far-reaching law–Proposition K, The Sunlight Ordinance–passed by voters in 1989. That ordinance, aimed at preventing the proliferation of shadows in already-chilly San Francisco, restricts any new construction that would block sunlight on a public park. (Among other things, the law gave birth to a unit of measurement that has probably never existed anywhere else in history: the “solar-year square-foot-hour of new shade.”) St. Mary’s is the place where the irresistible force of San Francisco’s development pressure met the immovable object of the arc of solar radiation.

Because of its location and open-space importance to workers and residents, St. Mary’s was given the most stringent designation under Proposition K: zero tolerance. The square could not have a single additional square-foot-hour of sunlight taken away. An office tower was proposed that would have blocked a small amount of sunlight–only in the early morning, four months out of the year–but that was enough to kill the proposal, until a wonderful solution was proposed. The developer offered to create a public park on the second-floor roof of the building’s garage. The 5,000-square-foot roof was located in such a way that it received much more sunlight; in fact, the developer calculated that the amount of sunlight hitting the rooftop park addition would be 40 times greater than the sunlight lost to the old park by the building’s shadow. And, because of the steepness of the site, the second-story location actually intersected with the ground plane of a portion of St. Mary’s Square, removing the need for steps or handicap accessible designs.

Overcoming the hurdles of rooftop park technology and getting people up off the ground can be challenging. But rooftop parks could make a big difference when it comes to urban beauty, livability, and recreation. This abundant resource beckons, particularly in extremely dense communities that are very short of parkland.